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[SNMP]: SONiC SNMP Changes to support IPv6 (#1457)
Document captures the changes required to support SNMP over IPv6 for single asic platforms. [SNMP][IPv6]: Fix SNMP IPv6 reachability issue in certain scenarios [SNMP][IPv6]: Fix SNMP IPv6 reachability issue in certain scenarios sonic-buildimage#15487 [SNMP][IPv6]: Fix to use link local IPv6 address as snmp agentAddress [SNMP][IPv6]: Fix to use link local IPv6 address as snmp agentAddress sonic-buildimage#16013 [SNMP]: Modify minigraph parser to update SNMP_AGENT_ADDRESS_CONFIG [SNMP]: Modify minigraph parser to update SNMP_AGENT_ADDRESS_CONFIG sonic-buildimage#17045
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| # SONiC SNMP Changes to support IPv6 # | ||
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| This document captures the changes required to support SNMP over IPv6. | ||
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| ## Motivation ## | ||
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| SNMP query over IPv6 address fails in certain scenarios on single asic platforms. | ||
| Ideally, SNMP query should be successful over both IPv4 and IPv6 addresses. | ||
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| ## Current configuration for SNMP ## | ||
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| Currently, snmpd process inside SNMP docker uses snmpd.conf as the configuration file. | ||
| One of the configuration directives in snmpd.conf is *agentaddress*. | ||
| *agentaddress* defines list of listening address on which SNMP request can be received. | ||
| In SONiC, the default listening address is 'any ip'. | ||
| ``` | ||
| agentAddress udp:161 | ||
| agentAddress udp6:161 | ||
| ``` | ||
| The other method is to use the below configuration command to configure agent address. | ||
| ``` | ||
| config snmpagentaddress add <ip> | ||
| ``` | ||
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| ## Issue seen with IPv6 ## | ||
| In case of SNMP query over IPv6, the SNMP query fails with timeout. | ||
| This SNMP query fails over IPv6 because SNMP response goes out with the incorrect SRC IP. | ||
| This SRC IP is incorrect in SNMP packet as snmpd does not keep track of the DST IP from the SNMP request packet. | ||
| Below is a packet capture showing SNMP request packet sent to DUT Loopback IPV6 address with SRC IP as port-channel IP of cEOS neighbor. The SNMP response packet goes out with SRC address as DUT port-channel IP whereas it should have been the DUT Loopback IPv6 address. | ||
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| ``` | ||
| 23:18:51.620897 In 22:26:27:e6:e0:07 ethertype IPv6 (0x86dd), length 105: fc00::72.41725 > fc00:1::32.161: C="public" GetRequest(28) .1.3.6.1.2.1.1.1.0 | ||
| 23:18:51.621441 Out 28:99:3a:a0:97:30 ethertype IPv6 (0x86dd), length 241: fc00::71.161 > fc00::72.41725: C="public" GetResponse(162) .1.3.6.1.2.1.1.1.0="SONiC Software Version: SONiC.xxx - HwSku: xx - Distribution: Debian 10.13 - Kernel: 4.19.0-12-2-amd64" | ||
| ``` | ||
| **Sequence of SNMP request and response** | ||
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| 1. SNMP request will be sent with SRC IP fc00::72 DST IP fc00:1::32 | ||
| 2. SNMP request is received at SONiC device is sent to snmpd which is listening on port 161 :::161/ | ||
| 3. snmpd process will parse the request create a response and sent to DST IP fc00::72. | ||
| 4. snmpd process does not track the DST IP on which the SNMP request was received, which in this case is Loopback IP. | ||
| snmpd process will only keep track what is tht IP to which the response should be sent to. | ||
| 5. snmpd process will send the response packet. | ||
| Kernel will do a route look up on destination IP and find the best path. | ||
| ip -6 route get fc00::72 | ||
| fc00::72 from :: dev PortChannel101 proto kernel src fc00::71 metric 256 pref medium | ||
| 6. Using the "src" ip from above, the response is sent out. This SRC ip is that of the PortChannel and not the device Loopback IP. | ||
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| The same issue is seen when SNMP query is sent from a remote server over Management IP. | ||
| SONiC device eth0 --------- Remote server | ||
| SNMP request comes with SRC IP <Remote_server> DST IP | ||
| If kernel finds best route to Remote_server_IP is via BGP neighbors, then it will send the response via front-panel interface with SRC IP as Loopback IP instead of Management IP. | ||
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| Main issue is that in case of IPv6, snmpd ignores the IP address to which SNMP request was sent, in case of IPv6. | ||
| In case of IPv4, snmpd keeps track of DST IP of SNMP request, it will keep track if the SNMP request was sent to mgmt IP or Loopback IP. | ||
| Later, this IP is used in ipi_spec_dst as SRC IP which helps kernel to find the route based on DST IP using the right SRC IP. | ||
| https://github.com/net-snmp/net-snmp/blob/master/snmplib/transports/snmpUDPBaseDomain.c#L300 | ||
| ipi.ipi_spec_dst.s_addr = srcip->s_addr | ||
| Reference: https://man7.org/linux/man-pages/man7/ip.7.html | ||
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| **Why should the SRC IP in SNMP response match the DST IP in SNMP request?** | ||
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| In snmp pdu, There is a request-id, and response has to match the request id. | ||
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| Reference RFC 1157 | ||
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| _In GetResponse-PDU, the value of the request-id field of the GetResponse-PDU is that of the received message._ | ||
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| **This issue is not seen on multi-asic platform, why?** | ||
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| On multi-asic platform, there exists different network namespaces. | ||
| SNMP docker with snmpd process runs on host namespace. | ||
| Management interface belongs to host namespace. | ||
| Loopback0 is configured on asic namespaces. | ||
| Additional inforamtion on how the packet coming over Loopback IP reaches snmpd process running on host namespace: #5420 | ||
| Because of this separation of network namespaces, the route lookup of destination IP is confined to routing table of specific namespace where packet is received. | ||
| If packet is received over management interface, SNMP response also is sent out of management interface. Same goes with packet received over Loopback Ip. | ||
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| ## Changes done to workaround the issue ## | ||
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| Currently snmpd listens on any ip by default, after this change: | ||
| 1. If minigraph.xml is used to load the initial configuration, then SNMP_AGENT_ADDRESS_CONFIG in config_db will be updated with Loopback0 and Managment interface IP addresses during the parsing of minigraph.xml. | ||
| 2. No change will be done if config_db.json is used to load the configuration. | ||
| config_db.json file could have SNMP_AGENT_ADDRESS_CONFIG table udpated with required IP addresses for snmpd to listen on. | ||
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| Before the change: | ||
| snmpd listens on any IP, snmpd binds to IPv4 and IPv6 sockets as below: | ||
| ``` | ||
| netsnmp_udpbase: binding socket: 7 to UDP: [0.0.0.0]:0->[0.0.0.0]:161 | ||
| trace: netsnmp_udp6_transport_bind(): transports/snmpUDPIPv6Domain.c, 303: | ||
| netsnmp_udpbase: binding socket: 8 to UDP/IPv6: [::]:161 | ||
| ``` | ||
| When IPv4 response is sent, it goes out of fd 7 and IPv6 response goes out of fd 8. | ||
| When IPv6 response is sent, it does not have the right SRC IP and it can lead to the issue described. | ||
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| If minigraph.xml is used, then SNMP_AGENT_ADDRESS_CONFIG will be configured with Loopback0 and Managment interface IP addresses. When snmpd listens on specific Loopback/Management IPs, snmpd binds to different sockets: | ||
| ``` | ||
| trace: netsnmp_udpipv4base_transport_bind(): transports/snmpUDPIPv4BaseDomain.c, 207: | ||
| netsnmp_udpbase: binding socket: 7 to UDP: [0.0.0.0]:0->[10.250.0.101]:161 | ||
| trace: netsnmp_udpipv4base_transport_bind(): transports/snmpUDPIPv4BaseDomain.c, 207: | ||
| netsnmp_udpbase: binding socket: 8 to UDP: [0.0.0.0]:0->[10.1.0.32]:161 | ||
| trace: netsnmp_register_agent_nsap(): snmp_agent.c, 1261: | ||
| netsnmp_register_agent_nsap: fd 8 | ||
| netsnmp_udpbase: binding socket: 10 to UDP/IPv6: [fc00:1::32]:161 | ||
| trace: netsnmp_register_agent_nsap(): snmp_agent.c, 1261: | ||
| netsnmp_register_agent_nsap: fd 10 | ||
| netsnmp_ipv6: fmtaddr: t = (nil), data = 0x7fffed4c85d0, len = 28 | ||
| trace: netsnmp_udp6_transport_bind(): transports/snmpUDPIPv6Domain.c, 303: | ||
| netsnmp_udpbase: binding socket: 9 to UDP/IPv6: [fc00:2::32]:161 | ||
| ``` | ||
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| When SNMP request comes in via Loopback IPv4, SNMP response is sent out of fd 8 | ||
| ``` | ||
| trace: netsnmp_udpbase_send(): transports/snmpUDPBaseDomain.c, 511: | ||
| netsnmp_udp: send 170 bytes from 0x5581f2fbe30a to UDP: [10.0.0.33]:46089->[10.1.0.32]:161 on fd 8 | ||
| ``` | ||
| When SNMP request comes in via Loopback IPv6, SNMP response is sent out of fd 10 | ||
| ``` | ||
| netsnmp_ipv6: fmtaddr: t = (nil), data = 0x5581f2fc2ff0, len = 28 | ||
| trace: netsnmp_udp6_send(): transports/snmpUDPIPv6Domain.c, 164: | ||
| netsnmp_udp6: send 170 bytes from 0x5581f2fbe30a to UDP/IPv6: [fc00::42]:43750 on fd 10 | ||
| ``` | ||
| This separation of socket fd will ensure that SNMP response goes out with the right SRC address. | ||
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| This change is also more secure approach instead of listening over any ip. | ||
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| ### Effects of this change ### | ||
| 1. If minigraph.xml is used to load the initial configuration, then snmpd will listen on Loopback0/Management IPs on single asic platform. | ||
| 2. If config_db.json is used load the configuration, then snmpd will listen on any IP, the IPv6 issue will be seen. To overcome IPv6 issue, SNMP_AGENT_ADDRESS_CONFIG table should be udpated to listen to specific IPv4 or IPv6 addresses using "config snmpagentaddress add <ip>". | ||
| 3. No change required or done for multi-asic platforms. | ||
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| ### Pull request to support this change ### | ||
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| 1. [SNMP][IPv6]: Fix SNMP IPv6 reachability issue in certain scenarios https://github.com/sonic-net/sonic-buildimage/pull/15487 | ||
| 2. [SNMP][IPv6]: Fix to use link local IPv6 address as snmp agentAddress https://github.com/sonic-net/sonic-buildimage/pull/16013 | ||
| 3. [SNMP]: Modify minigraph parser to update SNMP_AGENT_ADDRESS_CONFIG https://github.com/sonic-net/sonic-buildimage/pull/17045 |